$ E = \left[\begin{array}{r}1 \\ 3 \\ -1\end{array}\right]$ $ A = \left[\begin{array}{rr}0 & -1\end{array}\right]$ Is $ E A$ defined?
Solution: In order for multiplication of two matrices to be defined, the two inner dimensions must be equal. If the two matrices have dimensions $( m \times  n)$ and $( p \times q)$ , then $ n$ (number of columns in the first matrix) must equal $ p$ (number of rows in the second matrix) for their product to be defined. How many columns does the first matrix, $ E$ , have? How many rows does the second matrix, $ A$ , have? Since $ E$ has the same number of columns (1) as $ A$ has rows (1), $ E A$ is defined.